Equipment required for new VLF/Omega global navigation systems requires the production of sophisticated test apparatus for simulating the complex network of VLF (Very Low Frequency) and Omega signals to adequately test the operability of the equipment. In order to simulate these signals, precise phase control is necessary along with the capability of individual Doppler frequency phase shifting to simulate motion along a ground track. The present invention illustrates a new and simple approach to providing said phase control and Doppler shift with a minimum of components.
A digital clock is generated whose frequency is 2.sup.N times that of the desired signal. The digital clock increments an N-bit counter whose outputs are applied to address a sine look-up table PROM (Programmable Read Only Memory). The PROM outputs drive a digital analog converter which produces a step approximation to the desired signal. These steps are then removed by filtering to provide the desired sine-wave output signal. A phase offset may be obtained by presetting the N-bit counter to some given state by a synchronizing signal which is a sub-multiple of the desired output. Then a multiple number of such signal generators are connected to form the complex signal conditions under which the equipment must operate. There must be a synchronization of all the generators such that the phase relationship between all the signals can be precisely defined. This is accomplished by making the synchronizing signal a sub-multiple of all the signals that must be generated.
By incrementally changing the preset count of the phase counter over a period of time, the phase of the output signal will increase or decrease giving an apparent frequency shift.
The present system is more stable and uses fewer parts than prior art attempts at phase controlled signal generators using variable frequency VCO's (Variable Controlled Oscillator) and pulse stuffer and dropper techniques to achieve phase shift and Doppler frequency shift.
It is therefore an object of the present invention to provide an improved phase controlled signal generator.